ABSTRACT. The use of radiation during electrophysiology procedures is associated with both short- and long-term risk of radiation effects. Techniques to minimize radiation, most notably the incorporation of three-dimensional (3D) mapping technology, have led to a dramatic decrease in radiation exposure during electrophysiology procedures. However, 3D mapping technology is less suited for infants and small children due to the size of the navigation patches and catheters designed for adults. This report is the first to describe the use of 3D mapping in an infant, and the modifications needed to allow 3D mapping to be used in patients this size. This technique allowed a nearly radiation free procedure in an infant with tachycardia induced cardiomyopathy.

The authors report no conflicts of interest for the published content. , Manuscript received November 28, 2012, final version accepted December 21, 2012.

Introduction

The use of radiation during electrophysiology procedures is associated with deterministic and stochastic radiation-related injury. Of significant concern in infants and children is the long-term cancer-related risk, which appears to be amplified in this population.1 Adoption of techniques to minimize radiation exposure, most notably in the incorporation of three-dimensional (3D) mapping technology, have led to a dramatic decrease in radiation exposure during electrophysiology procedures, and in many cases have eliminated radiation exposure altogether. Unfortunately, due to small patient size, the relatively large catheter sizes and surface patch size to the infant body size, this technology has been less feasible in infants. This report describes an infant who presented after birth with atrial ectopic tachycardia recalcitrant to antiarrhythmic medications who developed tachycardia-induced cardiomyopathy. The use of the 3D mapping allowed successful ablation with almost no radiation exposure. This report is the first to describe the use of 3D mapping in an infant, and the techniques employed to adapt the system for this infant are described.

Case presentation

A 3.12-kg female infant was born at 37 0/7 weeks of gestation by emergent C-section after noting fetal tachycardia shortly before birth. The pregnancy was otherwise uncomplicated. After delivery, the infant exhibited incessant supraventricular tachycardia consistent with an atrial arrhythmia initially felt to be atrioventricular re-entrant or intra-atrial re-entrant tachycardia, with a heart rate of 210–250 bpm. Initial treatment included vagal maneuvers, multiple doses of adenosine, electrical cardioversion, and digoxin but the tachycardia persisted with little to no change in rate. The infant began to exhibit grunting and mildly poor perfusion; therefore, amiodarone was started. An echocardiogram showed a structurally normal heart with a normal ejection fraction and tachycardia. Over the next few weeks, in spite of many adjustments to the medicine regimen, the infant continued to exhibit supraventricular tachycardia (SVT) most consistent with an atrial ectopic tachycardia (AET). Antiarrhythmic mediations used included digoxin, propranolol, amiodarone, esmolol, and flecainide in various combinations. Though there were a few transient episodes of conversion to sinus rhythm, the AET was otherwise incessant. The infant exhibited medication side effects including a rate-related bundle branch block, which was felt to be secondary to the initiation of flecainide with amiodarone. However, after a period without flecainide the rate related bundle branch block remained. Serial echocardiograms showed a progressive decrease in left ventricular function. The ejection fraction was 40% on day of life 25 on amiodarone, flecainide and propranolol. On day of life 25–28, though the patient remained in AET, there appeared to be improved rate control with the heart rate ranging between 150 and 170 bpm. However, an echocardiogram on day of life 33 showed a significant decrease in the ejection fraction, below 20%, and the decision was made to proceed to an electrophysiology study and possible ablation.

On day of life 35, weighing 4.1 kg, the infant presented to the electrophysiology laboratory in a fasting state. The infant was intubated and sedated. Owing to a family history of malignant hyperthermia intravenous sedation with etomidate, fentanyl, and versed was given in place of inhaled anesthetics. The EnSite NavX (St. Jude Medical, St. Paul, MN) surface electrodes and ablation pads were modified to accommodate the small chest size (Figure 1). After placing the R2 and NavX patches, the surface leads and the ablation grounding patch (Figure 2) the infant was prepped and draped. Using the modified Seldinger technique, two venous sheaths (5Fr in the right femoral vein, and 4Fr in the left femoral vein) and one arterial 20-gauge quick catheter were placed. An esophageal catheter (Tapcath, Cardio Command Inc, Tampa, FL) was advanced behind the left atrium using 3D NavX, but the electrogram signal quality was suboptimal. Occasional flashes of fluoroscopy were used to confirm esophageal lead location and for wire location with sheath placement. Without fluoroscopy a steerable 4-Fr decapolar catheter (Irvine Biomedical, Irvine, CA) was advanced to the heart via the left femoral vein while monitoring with 3D NavX. The inferior vena cava and superior vena cava were marked by electrogram guidance and a right atrial and some right ventricular geometry was made by sweeping the catheter through the chamber (Figure 3). A patent foramen ovale was labeled on 3D NavX after passing through to the left atrium with the catheter. The steerable catheter was then placed across the tricuspid valve to enable recording and pacing at the right ventricle with the distal electrodes and recording of right atrial signals (passing near the His area) with the proximal electrodes. A 5-Fr quadripolar radiofrequency ablation catheter (Mariner, Medtronic, Minneapolis MN) was then advanced into the right atrium to measure atrial signals and eventually to perform the ablation. Baseline intracardiac electrograms were recorded during the ectopic tachycardia. There was no ventriculoatrial (VA) conduction with ventricular pacing. The 5-Fr ablation catheter was used to create an activation map (Figure 3) which identified an ectopic focus just on the atrial side of the tricuspid valve around an ∼11 o’clock position. The location of the His bundle was evaluated to confirm an appropriate distance from the AV node given the small heart size (for reference, the ablation lesions in Figure 3 are 3 mm in diameter). At the area of earliest activation there was a brief termination of the arrhythmia, followed by return of the AET. At this site a radiofrequency ablation was placed, initially at 50°C. There was termination of the AET at ∼1 s to a sinus rhythm at ∼120–130 bpm (Figure 4a,b). The ablation required only a few watts of energy to obtain the goal temperature of 50°C. The temperature was increased to 55°C once there was no evidence of AV nodal or SA nodal damage and a 60 s lesion was placed. A repeat 30-s lesion was placed on the same site as mapped by 3D NavX. An isoproterenol bolus (0.8 µg) was delivered without return of the AET. After further testing revealed no spontaneous or inducible arrhythmia the catheters were withdrawn, though femoral lines were kept until after transfer to the neonatal intensive care unit (NICU). During the case fluoroscopy had been used to assist with line placement, esophageal lead placement, and a brief image was taken to evaluate catheter positioning within the heart silhouette. In all 12 s and <1 mGy of fluoroscopy were used. The total procedure time was 1 h and 1 min from line placement to line removal.

Figure 1: The EnSite NavX patches (front and side) were modified to fit the infant. Also shown is the blue ablation patch. The back and left-side patches were cut to the same size as the corresponding patch shown here. A 10-cc syringe is included for a size reference.

Figure 2: The infant after placement of the R2 pads, the modified NavX patches, the electrodes and the ablation pad.

Figure 3: A right anterior oblique and slightly left anterior oblique view of the intracardiac geometry of the superior vena cava, the inferior vena cava, the right atrium and right ventricle generated with three-dimensional NavX. The white on the surface of the cardiac geometry at ∼ 11 o’clock on the atrial side of the tricuspid valve annulus is the activation map indicating the site of earliest atrial activation. The overlapping red and green circular lesions are located at the site of successful ablation, and measure 3 mm in diameter. The location of the His electrogram is labeled, and the green intracardiac catheter advancing from the right atrium to the right ventricle is shown. Unfortunately, as we did not spend additional time obtaining the full RV geometry, the RV appears somewhat skewed in the leftward view. TV: tricuspid valve. SVC: superior vena cava.

Figure 4: Intracardiac and surface electrograms of the atrial ectopic tachycardia at the time of ablation are shown. (a) The intracardiac electrograms with two beats of the atrial ectopic tachycardia and one beat after termination to sinus rhythm. (b) The surface electrograms of the same complexes.

After the procedure the infant returned to the NICU in a sinus rhythm and rate of ∼120 bpm. She was extubated shortly thereafter, and the femoral lines were removed. A brief echocardiogram showed no effusion and subjectively improving function. A full echocardiogram the following day showed a dramatic improvement in heart function, with an ejection fraction (EF) estimated at 44%, and normal left ventricular function (EF of 63%) the following day. She remained off all antiarrhythmia medications and was on full feeds, on room air and in sinus rhythm at discharged on day of life 37, just 2 days after the procedure. Approximately 1 month later she was found to have an episode of elevated heart rate consistent with SVT, presumably a recurrence of the arrhythmia. Interestingly, unlike the previous episodes, this had a sudden termination with adenosine. Therefore she infant was restarted on flecainide and has been well controlled with no arrhythmias since.

Discussion

As radiation is associated with stochastic events, probabilistic events with no known lower threshold of safety, the judicial use or elimination of radiation is especially important in the young. Young patients have increased risk from radiation exposure due to their longer lifespan, the close proximity of other organs to the radiation field, and a greater sensitivity to radiation which has been estimated at over four times higher in infants then in adults.1 Though it is difficult to estimate the true risk of a stochastic effect from radiation exposure, some of the most extensive work predicts a lifetime risk of development of a solid cell tumor in one in 100 people exposed to a dose of just 100 mSv of radiation.1 This is a dose lower than that received during many standard electrophysiology procedures.2,3,4

The use of 3D mapping systems has substantially reduced the radiation exposure in patients undergoing electrophysiology procedures. However, due to the size limitations associated with a small chest, the EnSite NavX surface electrode patches for positional reference require modification. The electrode patches consist of a central portion near the entrance of the wire at the base of the patch which cannot be modified, and the outer portion of the patch which may be trimmed. St. Jude described preliminary guidelines for cutting NavX patches to modify them for smaller patient size (Guidelines for Cutting NavX patches, published July 10 2003), though this is not in the current instruction manual. According to this information, cutting of the NavX patches can allow the patch size to be adjusted to a 4×4 inch patch for the front and back patches, and a 2.5×2.5 inch patch on the left, right, neck, and leg patches. Cutting should not be done to shorten the side of the patch with the electrode wire. Our patches were cut to just under 3¾×2¼ inch (chest/back), and 2½×1½ inch (neck/leg, sides), smaller than the recommended size, with care taken to keep the opposite patches (front/back, neck/leg, right/left) equal in size (Figures 1 and 2) We were careful to avoid cutting the central portion near the wires. Additionally, the patches should be oriented opposite one another (for example if the anterior chest surface patch is to the left of the patients sternum, the posterior patch should also be to the left of the patients spine), and the patches should not overlap other electrodes, though side to side contact should not interfere.5 The patches appeared to function well in our patient.

In our procedure a small amount of radiation was used (12 s total) to confirm appropriate vascular access and to assist with location of the esophageal lead. To allow accurate 3D map creation of the intracardiac geometry without radiation electrograms are necessary to localize the catheter position. However, as we placed the esophageal lead first and as it had inadequate signals, radiation was used to help assess the esophageal lead location. After obtaining intracardiac geometry with the 4-Fr decapolar steerable catheter, we were able adjust the esophageal lead in reference to that geometry. Because the small size of our patient, we used smaller French catheters, then often used in our electrophysiology studies.

Conclusion

This report is the first to describe a 3D mapping system to almost eliminate radiation in an electrophysiology procedure of an infant. This report highlights the feasibility of this technology to adapt to patient size and challenges the interventional electrophysiologists to minimize radiation exposure in this vulnerable population.